This invention is concerned with the removal of metals or metalloids, arsenic in particular, from groundwater, surface water, or other contaminated waters. More specifically, this invention is concerned with arsenic removal via complex-adsorption media from a water pre-treated with carbon dioxide to lower the pH of the water.
Water can be supplied to a treatment system from groundwater sources, surface water sources or from various types of wastewater supply. Often, the water supplied for treatment is contaminated with metals or metalloids to a greater or lesser degree. Representative metals are: copper, antimony, selenium, mercury, cadmium, lead and chromium. Arsenic (arsenate) and phosphorous (phosphate) are representative metalloids.
Arsenic is a naturally occurring metalloid found in many groundwater supply systems. Additionally, arsenic is found in wastewater or other contaminated water from numerous sources. Arsenic has been associated with several forms of cancer and other non-cancer illnesses when ingested. The EPA has promulgated a new arsenic Maximum Contaminant Level (MCL) of 10 xcexcg/L in drinking water. It is estimated that at an arsenic MCL of 10 xcexcg/L approximately 5% of the water systems in the U.S. will be out of compliance (Frey, 1999). The majority of those systems are groundwater systems located in the southwestern U.S.
It has been estimated that the national cost of compliance at an arsenic MCL of 10 xcexcg/L is in the range of $2.5 to 4 Billion. In the state of New Mexico alone, the capital cost for arsenic treatment at an MCL of 10 xcexcg/L may exceed $400 Million.
Two known complex-adsorption methods of removing arsenic from groundwater are by adsorption on to a fixed media or adsorption on a coagulant followed by filtration or microfiltration. The fixed media may be one of many including granular ferric hydroxide, activated alumina, iron modified activated alumina, iron modified zeolite, and other proprietary media. Coagulants may include ferric chloride, aluminum sulfate, copper sulfate, or ferric sulfate. The adsorption of arsenic on a media or a coagulant relies on surface complexation to remove arsenic from water, that is, the media or the coagulant has a net positive charge which allows complexation with negatively charged arsenate molecules. The degree of positive charge on the media or coagulant is a function of feed water pH. As the pH is lowered, the adsorption media or coagulant will have a greater positive charge and a greater affinity for the negatively charged arsenate molecules. This complex-adsorption process will apply to other metalloids and metals as well.
With many fixed adsorptive media, the life (or xe2x80x9crun lengthxe2x80x9d) is very short at raw water pH values greater than 8.0. As such, the economic viability of adsorption media is very poor at pH values greater than 8.0. If the pH of the feed water can be reduced, adsorption media will last longer and will be more economical. Adding a mineral acid such as sulfuric acid or hydrochloric acid can reduce the pH, however these strong acids are dangerous and hazardous to store and handle. In addition, the low pH water will be corrosive to the distribution system and will require the further addition of a strong base such as caustic soda to increase the pH. Therefore, one may consider the use of mineral acids and strong bases when implementing an adsorption media and the raw water pH is greater than 8.0, but with additional cost and safety concerns. In addition, groundwaters that have a high silica concentration may cause adsorption media and microfilter membranes to foul at higher pH values(Chen, et al., Predicting Arsenic Removal by Ferric Hydroxides in the Presence of Silica, Sulfate, and NOM, AWWA Conference, Jun. 19, 2001). Furthermore, coagulants such as ferric chloride exhibit better adsorption performance at lower pH values (Chwirka, et al, Journal of the American Water Works Association, March, 2000).
Several processes have been patented or have applications pending concerning the treatment of water supplies contaminated with arsenic. These prior art processes recognize the necessity of adjusting the pH of an arsenic containing solution or oxidizing the arsenic to achieve desired removal strategies. For example, Gallup, U.S. Pat. No. 5,024,769, which is primarily concerned with the treatment of geo-thermal brine which has a relatively high proportion of arsenite wherein the arsenic is in the +3 valance state, teaches contacting of the arsenic containing solution with an oxidizing agent, specifically an oxidizing biocide. This process oxidizes a substantial portion of the arsenite compounds to arsenates. The oxidation of the arsenic compounds to arsenate aids in its subsequent reaction to form an insoluble precipitate.
McLintock, U.S. Pat. No. 5,358,643 discloses the treatment of water containing arsenic with an acid and an oxidant to facilitate arsenic removal. Sulfuric acid is specifically mentioned as the preferable acid for the McLintock process. McLintock also teaches the use of strong base such as sodium hydroxide to raise pH and facilitate precipitation after arsenic complexion. Similarly, Golden, published U.S. application U.S. 2002/0003116 A1 discloses the use of an acid to depress pH followed by treatment with hydrogen peroxide (an oxidizing agent) to enhance arsenic removal from water. Although Golden does not specify a preferred acid, the acidification step requires a pH of 3-5 which will only be achieved by the addition of a relatively concentrated and strong acidifying agent.
Each of the methods known in the prior art which compensate for the pH dependence of arsenic removal by complex-adsorbing means rely on relatively dangerous and hazardous chemicals to accomplish the needed pH and oxidation state adjustments.
The present invention is intended to overcome one or more of the problems discussed above.
A first aspect of the present invention is a method of removing a metal or metalloid from water which includes providing water containing a metal or metalloid from a source. The water is contacted with carbon dioxide to lower the pH of the water resulting in a pre-treated water. The pre-treated water is then contacted with a metal or metalloid complex-adsorbing substance to produce a metal or metalloid depleted water. Carbon dioxide is then removed or stripped from the depleted water to increase the pH of the depleted water. The carbon dioxide is preferably stripped from the depleted water by some form of aeration. The metal or metalloid may be selected from the group consisting of arsenic, copper, antimony, selenium, mercury, cadmium, chromium and lead. The pH is preferably lowered to about 7 or less to produce the pre-treated water.
In one embodiment the metal or metalloid complex-adsorbing substance is an adsorption media. In another embodiment the metal or metalloid complex-adsorbing substance is a coagulant and following contacting treatment, the depleted water is filtered to separate the coagulant and bound metal or metalloid from the depleted water.
Another aspect of the present invention is a method of removing arsenic from water which includes providing water containing arsenic from a source. The water is contacted with carbon dioxide to lower the pH of the water and produce a pre-treated water. The pre-treated water is contacted with an arsenic complex-adsorbing substance to produce arsenic depleted water. The pH is preferably lowered to about 7 or less to produce the pre-treated water. The arsenic depleted water is preferably subject to stripping of the carbon dioxide to increase the pH of the arsenic depleted water. The stripping may be accomplished by aeration. The complex-adsorbing substance may be an adsorption media or a coagulant. When the complex-adsorbing substance is a coagulant a further step of filtering the depleted water to separate the coagulant and the bound arsenic from the depleted water is added. Filtering can be accomplished, for example, with microfilter, conventional filtration, pressure filtration, AquaDisk filter, or other filtration device. In one embodiment, the pre-treated water is contacted with the arsenic complex-adsorbing substance for a time sufficient to produce an arsenic depleted water having less than 10 xcexcg/liter of arsenic.
Yet another aspect of the present invention is an apparatus for removing arsenic from water containing arsenic. The apparatus includes a water flow from a source of water containing arsenic. A source of carbon dioxide is provided in fluid communication with the water flow to supply carbon dioxide to the water flow to thereby lower the pH of the water producing a pre-treated water flow. A contact chamber containing an arsenic complex-adsorbing substance is provided in fluid communication with the pre-treated water flow to remove arsenic from the pre-treated water flow producing a depleted water. A carbon dioxide stripping apparatus may be provided in fluid communication with the depleted water flow for stripping carbon dioxide from the depleted water. The contact chamber may be an adsorption chamber containing an adsorption media. Alternatively, the contact chamber may be a mixing tank in fluid communication with a coagulant supply system supplying an arsenic complex-adsorbing coagulant to the pre-treated water flow. In this embodiment, a filter is provided downstream of the contact chamber to filter the coagulant and yield a filtered water flow.
The method and apparatus of the present invention allows for the removal of arsenic and other metalloids and metals from a source of water. Use of carbon dioxide to lower the pH of the water prior to the arsenic removal enhances the efficiency of the removal and extends the life of the metal or metalloid complex-adsorbing substance used to remove the arsenic. Subsequent removal of the carbon dioxide by aeration or other means raises the pH of the water so that it will not present a corrosion risk following the treatment process. The use of carbon dioxide to adjust the pH eliminates storing and handling of hazardous strong acids and bases and eliminates the addition of chemicals which can adversely effect the taste or wholesomeness of the treated water. This method does not add total dissolved solids (xe2x80x9cTDSxe2x80x9d) to the water, as the use of strong acids and bases would. The method and apparatus can be used to remove any metal, metalloid or non-metals such as dissolved organic matter where removal by a complex-adsorbing media is pH dependent. Moreover, the method and apparatus are simple and result in overall cost savings.